I've found them at the 3 biggies... TRU, Target, WM... I've found more at WM, next Target, and a few at TRU here and there though mostly bought for me by friends.
The KB's are all in malls here so I rarely see what they have, but given that EVERY retailer is $6.99 (or thereabouts), KB doesn't look bad to me either... Nor does K-Mart who've kept fairly routine stock usually a wave behind or so (but good for getting stuff you miss) and are charging $6.99 like the rest of the ******** retailers.
I've never found large #'s of Sandtroopers at once but small dribs and drabs, yes.